Information records containing video, audio and color information in the form of a very fine surface relief pattern which can be read capacitively are known. The surface relief pattern is monitored capacitively by a playback stylus and the surface variations are reconstituted in electrical signal form and converted back to information suitable for display by a television receiver.
In a system described by Clemens in U.S. Pat. No. 3,842,194, incorporated herein by reference, a capacitive system is employed wherein the disc and the playback stylus are electrically conducting and it is essential that a dielectric layer be interposed between them. Initially, the disc was of plastic coated with a metal layer which was in turn coated with a thin layer of dielectric. Fox et al in copending application "A Conductive Video Disc," Ser. No. 105,550, filed Dec. 20, 1979, describe improved high density information discs prepared by compression molding a plastic molding composition containing a sufficient amount of conductive particles, such as carbon black, so that discs can provide capacitance readout.
The composition of Fox et al is comprised of a homopolymer or copolymer of vinyl chloride containing conductive particles and stabilizers, lubricants and processing aids. The conductive capacitive information disc records made from such compositions may utilize either a grooved or non-grooved information disc record system.
Improvements in the original formulations of Fox et al are disclosed in Khanna, U.S. Pat. No. 4,151,132 and Martin et al U.S. Pat. No. 4,228,050. However, these improved formulations, like those of Fox et al, are based on vinyl chloride homopolymers or copolymers. Products produced from these resins have a tendency to be moisture sensitive, both during processing and upon storage under ambient conditions, particurlarly in a hot, damp climate. In addition, such resins, e.g. PVC, are thermally unstable and will decompose during processing at elevated temperatures, liberating hydrochloric acid. The hydrochloric acid will react with other constituents of the formulation to form, e.g. hydroscopic salts and organic reaction products which may have a deleterious effect on the playback performance of the video disc. Further, PVC, after it has undergone processing at elevated temperatures, has a tendency to slowly decompose liberating hydrochloric acid which can cause reactions on the surface of the video disc that will interfere with the quality of playback. Because of these problems, all of the above-mentioned formulations have numerous ingredients which are balanced to create a product with acceptable workability, performance and stability. There exists, however, a need for a more stable conductive molding composition for video discs which contains substantially fewer components and is at least comparable to the present formulation in processing and performance. Such formulations are provided in accordance with this invention.